A rare-earth-iron-boron (hereinafter “RFeB”) magnet, which was discovered by Sagawa (the inventor of the present invention) et al. in 1982, is characterized in that its properties are far superior to those of the previously used permanent magnets and yet it can be produced from relatively abundant, inexpensive materials, i.e. neodymium (a rare-earth element), iron and boron. Due to these merits, this magnet is currently used in various products, such as the voice coil motors for hard disk drives or similar devices, drive motors for hybrid cars or electric cars, motors for battery-assisted bicycles, industrial motors, high-quality speakers, head phones, and magnetic resonance imaging (MRI) apparatuses using permanent magnets.
Three methods have been known to be available for producing RFeB magnets: (1) a sintering method; (2) a method including the steps of casting, hot working and aging treatment; and (3) a method including the step of die upsetting of a quenched alloy. Among these methods, the sintering method is superior to the other two in terms of magnetic properties and productivity and has already been established on the industrial level. With the sintering method, a dense, uniform and fine structure necessary for permanent magnets can be obtained.
Patent Document 1 discloses a method for producing an RFeB magnet by a sintering method. A brief description of this method is as follows: Initially, an RFeB alloy is created by melting and casting. This alloy is pulverized into fine powder and filled into a mold. A magnetic field is applied to this alloy powder, while a pressure is applied to the powder with a pressing machine. In this manner, both the creation of a compressed compact and the orientation of the same compact are simultaneously performed. Subsequently, the compressed compact is removed from the mold and sintered by heat to obtain an RFeB sintered magnet.
Fine powder of an RFeB alloy is easily oxidized and can ignite by reacting with oxygen in air. Therefore, the previously described process should preferably be performed entirely in a closed container whose internal space is free from oxygen or filled with inert gas. However, this is impractical because creating the compressed compact requires a large-sized pressing machine capable of applying a high pressure of 400 kgf/cm2 to 1000 kgf/cm2 to the alloy powder. Such a pressing machine is difficult to be set within a closed container.
Patent Document 2 discloses a method for producing a sintered magnet without creating a compressed compact. This method includes the three processes of filling, orienting and sintering, which are performed in this order to create a sintered magnet. A brief description of this method is as follows: In the filling process, an alloy powder is supplied into a filling container, after which this container is covered with a lid. For this filling container with the lid, a tapping operation is repeated to compact the alloy powder in the container. In the orienting process, a pulsed magnetic field is applied to orient the alloy powder in the filling container with the lid in one direction. Unlike the technique disclosed in Patent Document 2, no pressure is applied to the alloy powder during this magnetic orienting process. Therefore, the particles of the alloy powder repulse each other due to the applied magnetic field, causing an increase in the volume of the powder. However, since the filling container is covered with the lid, the powder volume cannot exceed the capacity of the container. In the sintering process, the alloy powder which has been oriented in one direction in the orienting process is sintered by heat together with the filling container covered with the lid. By this method, since no pressure is applied to the alloy powder in the magnetic orienting process, the particles of the alloy powder undergo no restrictions in their orienting motion, so that an RFeB magnet with even higher magnetic properties can be obtained.
Patent Document 2 also discloses an apparatus for producing a sintered magnet using a closed container whose internal space is free from oxygen or filled with inert gas, in which a filling unit, an orienting unit and a sintering unit are provided together with a conveyer for moving the filling container from the filling unit to the orienting unit and then from the orienting unit to the sintering unit. In this apparatus, the alloy powder is handled under an oxygen-free or inert-gas atmosphere throughout the entire process, so that the oxidization of the powder and the deterioration of magnetic properties due to the oxidization will not occur.